A bidirectional DC-DC converter is an electrical device widely applied to uninterruptible power supplies. An input end of a bidirectional DC-DC converter is connected to a rechargeable battery, and an output end of the bidirectional DC-DC converter is connected to positive and negative direct-current buses in an uninterruptible power supply. The bidirectional DC-DC converter may boost the voltage of a direct current of the rechargeable battery and then provide it to the positive and negative direct-current buses; or may use a direct current on the positive and negative direct-current buses to charge the rechargeable battery.
The voltage between positive and negative direct-current buses in uninterruptible power supplies commonly used now is about 700 volts. High-voltage battery boxes and low-voltage battery boxes are now provided on the market. For example, the output voltage of high-voltage battery boxes is 320 V to 550 V, and the output voltage of low-voltage battery boxes is 140 V to 320 V. When a high-voltage battery box is selected for use, a boost ratio of the bidirectional DC-DC converter is between 1.273 and 2.188 (less than 2.5), thus achieving high conversion efficiency. However, when a low-voltage battery box is selected for use, a boost ratio of the bidirectional DC-DC converter is between 2.188 and 5, causing decrease of the conversion efficiency. In order to solve the problem of excessively large boost ratio, two-stage boosting needs to be adopted, and another bidirectional DC-DC converter is usually connected between the output end of the bidirectional DC-DC converter and the positive and negative direct-current buses.
On one hand, another bidirectional DC-DC converter increases costs of the uninterruptible power supply. On the other hand, when a high-voltage battery box is selected for use, the added bidirectional DC-DC converter becomes redundant.